1. Field of the Invention
The invention concerns a method for analyzing a sample by sputtering by means of a particle beam, and to a device for the implementation of this method.
2. Description of the Prior Art
There are several known methods in the prior art for the chemical analysis of solids, using the process of sputtering by means of a particle beam. For example, in secondary ion mass spectrometry, particles are liberated from a sample to be analyzed by a beam of ions, called a primary beam, which scans the surface of the sample so as to hollow out a flat-bedded crater, and the materials liberated from the flat bed of the crater are identified by mass spectrometry.
To perform a reliable in-depth analysis of a solid sample, it is necessary to obtain a flat-bedded crater created by sputtering to form a surface to be analyzed or "surface of analysis" with a well-defined level. Generally, the primary beam is a continuous beam. For certain special applications, such as the analysis of giant molecules, a pulsed primary beam is used to enable a measurement of mass spectra by the time-of-flight technique.
More recently, new techniques have been used. These techniques consist in an additional post-ionization of the neutral particles, liberated by the primary beam, by means of a laser beam. This laser beam should be relatively high powered, and this implies the use of a pulsed laser beam. The primary beam is a beam of ions or photons produced by a continuous source, and this beam is chopped up into pulses by applying a pulsed voltage to deflection plates placed in the path of the particles between the source and the sample. The pulses of the primary beam are synchronized with the pulses of the laser in such a way that the laser pulses ionize the neutral particles liberated from the ample immediately after they have been liberated by the pulse of the primary beam.
Known methods of this type have been described, for example, in the following reference documents:
Hurst et al., Sputter Initiated Resonance Ionization Spectrometry, U.S. Pat. No. 4,442,354, Apr. 10, 1984; PA0 J. E. Parks et al., Thin Solid Films 8 (1983) 69; PA0 F. M. Kimock et al. Anal. Chem. 56 (1984) 2782; PA0 M. J. Pellin et al., Surf. Science 144 (1894) 619; PA0 C. Becker et al., Anal. Chem. 56 (1984) 1671.
Two problems are encountered in the implementing of these known methods.
A first problem concerns the time needed to make an analysis when a certain quantity of material of the sample has to be liberated, for example to make an in-depth profile measurement of the concentration of certain impurities in the sample. This is a problem because there is an incompatibility between the efficiency of an analysis and the speed with which this analysis is conducted. The post-ionization of neutral particles liberated from the sample is done by means of a pulsed laser beam because of the requirement for a relatively high-powered laser. The frequency of power laser pulses generally ranges between 10 and 200 Hz. The particles liberated by the primary beam in the interval between two consecutive pulses are clearly not all ionized and are therefore not detected. In cases where high detecting efficiency is necessary, information loss of this type is not acceptable. For this reason the primary beam is pulsed, in synchronism with the laser beam.
To obtain maximum detection efficiency, there would have to be only one pulse of the primary beam before each laser pulse. This would make the analysis very slow. For example, if the primary beam had pulses with a duration of one microsecond and a frequency of 100 Hz, its pulse ratio would be 10.sup.-4. Since the speed of analysis is directly proportionate to the sputtering speed, the speed of the analysis in this case would be smaller, by several orders of magnitude, than the speed obtained in the technique of secondary ion mass spectrometry without post-ionization, using a continuous primary beam. The measurement of a concentration profile up to a depth of one micron would take many hours. A period of time such as this is unacceptable.
To reduce the period of analysis, a known method consists in using a continuous primary beam to sputter the surface of the sample up to a certain depth, without seeking to make measurements of concentration. Then, the primary beam is chopped up into pulses which are synchronous with the pulses of the post-ionization laser beam, when the chosen depth is reached, in order to make a measurement of concentration at this depth. Then, a continuous primary beam is again used to sputter the sample until a subsequent level of depth where another measurement is made. Thus, going from level to level, this method enables an in-depth profile measurement of the concentration of impurities, while reducing the period of analysis to the minimum. However, it is clear that this measurement of the profile gives no information between each level of depth.
The aim of the invention is to propose a method of analysis by which the period of analysis can be reduced to the minimum without losing any substantial quantity of information.
A second problem relates to the making of the analyzer device, for this analyzer device should include a device for chopping the primary beam, with the following performance characteristics:
it should make it possible to obtain any pulse shape, and the pulse should be capable of being directed towards any point of the surface of the sample;
the surface of impact of the primary beam should stay the same position on the sample when the beam is on and when it is being turned off, i.e. it should not undergo any translational movements on the surface of the sample, under the effect of the voltage pulses controlling the deflection of the beam to turn it on or off;
the current of the primary beam reaching the sample should be strictly zero between the pulses of the primary beam, i.e. it should be totally off.
A second aim of the invention, therefore, is to propose an analyzer device to implement the method according to the invention while, at the same time, obtaining the above-mentioned performance characteristics